You’ve probably seen it before — a pond or lake with water that looks thick and greenish, almost like someone spilled paint in it. That’s often cyanobacteria, also called blue-green algae. And it’s not something to mess around with.
Here’s the scary part: according to the CDC, animals can get very sick or die within hours of swallowing toxins made by cyanobacteria. Dogs are especially at risk because they swim in whatever water they find and then lick themselves off afterward. By the time you realize something is wrong, it can already be too late.
Cyanobacteria produce two main types of toxins — ones that attack the nervous system (neurotoxins) and ones that destroy the liver (hepatotoxins). Not every bloom is toxic, but there’s no way to tell just by looking. In humans, exposure can cause abdominal pain, diarrhea, and vomiting. Liver poisoning symptoms might not show up right away either — the Washington State Department of Health notes it may be hours or even days before signs of liver damage appear, which makes it extra dangerous because you might not connect the dots in time.
Stagnant water during hot, dry weather is the biggest red flag. That’s when cyanobacteria really thrive and concentrations of toxins spike. If the water looks discolored, foamy, or has a thick scummy layer on the surface — stay out. Don’t swim in it, don’t let your dog run into it, and definitely don’t drink from it.
The rule is simple: when in doubt, stay out. Keep pets and livestock away from any water that looks even slightly off. If your dog has been in suspicious water and starts acting lethargic, vomiting, or having muscle tremors or seizures, get to a vet immediately. Don’t wait to see if they improve.